The invention relates to porous three-dimensional (3D) fiber structures made of refractory fibers. In particular it relates to such structures for constituting fiber preforms for making parts out of thermostructural composite material.
A particular field of application of the invention is the manufacture of friction parts, such as brake disks, having a carbon fiber reinforcement.
Thermostructural composite materials are well known for their good mechanical properties which make them suitable for constituting structural elements, and for their ability to retain these properties at high temperatures. They comprise in particular carbon/carbon (C/C) composite materials having carbon fiber reinforcement densified by a matrix of carbon, and ceramic matrix composite (CMC) materials having refractory fiber reinforcement (carbon fibers or ceramic fibers) densified by a matrix of ceramic material. Thermostructural composite materials find applications in particular in the fields of aviation and space, and in the field of friction, specifically for aircraft brake disks.
Composite material parts are usually fabricated by making a fiber preform which is to constitute the fiber reinforcement of the composite material, and by densifying the preform with the matrix of the composite material.
The preform is a porous fiber structure made up of fibers, yarns, or tows. Typical methods for making 3D fiber structures enabling preforms to be obtained having shapes that structures enabling preforms to be obtained having shapes that correspond to the shapes of the composite material parts that are to be fabricated include the following:                winding on a former or mandrel;        making thick felts;        three-dimensional weaving, knitting, or braiding; and        draping or superposing plies made of two-dimensional (2D) fabric and optionally bonded to one another; the 2D fabric may be a woven cloth, a sheet of unidirectional filaments, a multidirectional sheet made up of a plurality of unidirectional sheets superposed in different directions and bonded together, e.g. by needling or stitching, a layer of felt, or indeed a laminate comprising a cloth or sheet coated in a layer of free fibers that are bonded to the cloth or sheet by needling.        
A typical method of making a 3D fiber structure intended specifically for preparing preforms for thermostructural composite material disks consists in superposing plies made of layers of cloth or multidirectional sheet and in bonding them together by needling. One such method is described in particular in document U.S. Pat. No. 4,790,052. The fiber structure can be made from fibers that constitute precursors of the carbon or ceramic material constituting the fiber reinforcement of the composite material. The precursor is then transformed into carbon or ceramic by heat treatment, prior to being densified by the matrix.
The fiber structure may alternatively be made directly out of carbon fibers or ceramic fibers. In which case, the layers of cloth or multidirectional sheet can be provided with a layer of free fibers constituting a source of fibers suitable for being taken by the needles while the plies are being needled together.
A fiber preform can be densified with its matrix by a liquid process and/or a gaseous process. The liquid process consists in impregnating the preform with a liquid composition containing a precursor of the matrix and in subsequently transforming the precursor by heat treatment so as to obtain the desired matrix material. Typically, the precursor is a resin. The gaseous process is chemical vapor infiltration (CVI). The preform is placed in an oven into which a gas is admitted containing one or more components forming a gaseous precursor for the matrix material. The conditions inside the oven, in particular temperature and pressure, are adjusted so as to enable the gas to diffuse into the pores of the preform and deposit matrix material on the fibers of the preform, either by a component of the gas decomposing, or else by a reaction between a plurality of components. Such methods are well known in themselves, both for forming carbon matrices and for forming ceramic matrices.
Proposals have already been made to introduce fillers into the pores of fiber structures that are to constitute the fiber reinforcement of thermostructural composite material parts prior to densifying the fiber structures with the composite material matrix. The intended purposes are to reduce the fraction of the volume of the fiber structure that is represented by the pores in order to reduce the time taken by densification, and/or to impart special properties to the resulting composite material parts, and in particular to improve mechanical properties.
The fillers consist in particular of powders or of short fibers made of carbon or ceramic. A known method of introducing fillers consists in impregnating the 3D fiber structure with a suspension of fillers in a liquid. Nevertheless, it is found in practice to be impossible to distribute the fillers in desired manner within fiber structures, i.e. in uniform manner or in a manner that is not uniform but that is controlled.